Cathode ray tube



Sept. 5, 1939. G. F. BRETT ET AL 2,171,970

CATHODE RAY TUBE Filed Jan. 7, 1937 ELECT/30A! GUN YGEDfRGBALfV/A/ BANKS Patented Sept. 5, 1939 UNITED STATES PATENT OFFICE CATHODE. RAY TUBE aware Application January 7,

1937, Serial No. 119,475

In Great Britain January 24, 1936 2 Claims.

This invention relates to cathode ray tubes and more particularly to cathode ray tubes of the fluorescent screen kind suitable for use for television reception and for oscillograph purposes generally.

It is often desired in a cathode ray tube, particularly in a cathode ray tube for television reception, to provide a highly positive electrode at the far or large end of the tube in order to obtain a high electron velocity at the fluorescent screen and to this end it has been proposed to constitute the fluorescent screen by a deposition of fluorescent material upon a metal plate or disc at the large end of the tube. One defect of this arrangement is, however, that in order that the picture or pattern produced upon the fluorescent screen may be visible it is necessary to arrange the plate carrying the fluorescent screen at an angle to the axis of the tube so that the said pic- 20 ture or pattern may be seen through the side wall of the tube. This defect can, as is known, be avoided by depositing the layer of fluorescent material upon a thin silvering provided on the inner face of the said large end wall of the tube, the 25 silvering being sufficiently thin for the picture or pattern to be seen when looking through the said silvering. This known construction though reasonably satisfactory, has the disadvantage that there is a considerable amount of loss of 30 light by absorption and/or reflection in the silvered surface because, of course, light from the fluorescent material has to pass through the silvering and is therefore partially absorbed or reflected by the said silvering.

The present invention seeks to provide an improved cathode ray tube of the fluorescent screen kind which shall have the advantages of the known construction last described but in which there shall be a reduction of loss of light by absorption or reflection.

According to this invention the fluorescent screen material in a cathode ray tube of the kind referred to is deposited upon a thin conductive layer of a material whose color when viewed by transmitted light, matches or approximately matches the light emitted by the fluorescent material when activated. The fluorescent screen material may be deposited upon a conductive layer which is in turn deposited on the end wall of the 50 tube or alternatively the metal layer carrying the fluorescent layer may be deposited upon a mica or similar transparent screen member separate from the envelope of the tube but mounted in a suitable position inside the envelope.

If the fluorescent material is Willemite, or

some other substance which fluoresces with a greenish light, the thin conductive layer upon which the said material is deposited and through which the reproduced pictures or patterns may be viewed, is preferably of gold. A fine deposit of gold appears green when viewed by transmitted light and Willemite and certain other fluorescent substances emit a greenish light when activated. Accordingly by using a fluorescent screen made of Willemite deposited upon a thin gold covering provided on the inside of the large end wall of a cathode ray tube, the result may be achieved that the color of the light emitted by the fluorescent material substantially matches the color of the gold when viewed by transmitted light, and accordingly there will be little loss of light by reflection or absorption. Furthermore, gold surfaces, though very thin, are of good conductivity and have all the necessary properties for satisfying the requirements of an accelerating electrode in a cathode ray tube.

In carrying out this invention the thickness of the metal layer should, of course, be chosen with regard to the fiuorescing color of the fluorescent material.

There are various known methods available for obtaining a gold deposit upon the end of a cathode ray tube; e. g. such a gold film may be obtained by evaporation in vacuo, by sputtering, or by chemical methods.

The invention is, of course, not limited to the use of a gold metal layer for the choice of the material of the conductive layer and the thickness thereof will depend upon the fluorescing color of the fluorescent material employed. Gold is, however, probably the most satisfactory metal for use in association with Willemite.

One way of making the necessary connection for applying positive potential to the gold or other conductive layer at the end of the tube is to provide a platinum or other suitable wire, fused through the end wall of the tube and making contact with the gold. Another way of making such connection is as follows: the interior of the tube envelope is first silvered over from the neck portion of the tube down to the flat or screen end, but excluding the said screen end. The gold or other conductive layer is then deposited on the interior of the screen end of the tube so as to overlap somewhat the periphery of the silvering at that end. The fluorescent material is then deposited on the gold or other semitransparent conductive layer. With this construction connection can be made via the silvering to the gold or other backing to the fluorescent screen and since the silvering may be made quite thick and robust very certain and complete contact may be obtained. Moreover it is easy to make external connection to the silvering which may also serve to collect secondary electrons (if any) and as part of the final accelerating electrode of the tube.

In one way of carrying out the invention a cathode ray tube, which may have an envelope of any desired shape, for example the conventional approximately conical shape, is provided with a fluorescent screen the support memberfor which is constituted by a disc of thin transparent mica of about the same diameter as the screen end of the tube envelope. This mica is thin enough to permit of its being folded or bent suificiently to be inserted into the tube via the neck. Before any fluorescent material is deposited upon this disc of mica it is first given a deposit of the metal chosen for the thin conductive layer. For example of the fluorescent material is to be Willemite, the disc is painted or rubbed over with so-called liquid gold. Where the thin conductive layer is to be gold certain of the known zinc sulphide preparations, which fiuoresce with a greenish blue light well suited to the greenish blue transparency of thin gold, may be used as the fluorescent material. The mica is then heated for a few minutes to burn off organic matter, leaving a thin deposit of gold. A piece of nickel foil attached to a Wire is then slipped to the periphery of the mica so as to make good electrical contact with the gold. The lay of Willemite or other fluorescent material is then deposited on top of the gold.

The screen end of the envelope of the tube is provided with a hole at or near its periphery and the wire attached to the mica disc is pushed through this hole from the inside of the envelope, the said wire being inserted in the neck end of the envelope, which, at this stage of manufacture, is still open. The mica with its layer of gold and superimposed fluorescent layer is now bent or folded and inserted into the neck of the tube, being pulled along the tube into its proper position at the end thereof by the wire. When the mica is in position with the fluorescent side thereof facing the neck end of the tube, the aperture through which the wire passes is sealed off leaving a connection which can be connected to a point at any desired potential.

For further rigidity the periphery of the mica disc is preferably provided with one or more small spring members which press against the sides of the envelope of the tube.

The manufacture of the tube may then be completed in the ordinary well known way.

Our invention will best be understood by referring to the following drawing, in which Fig. 1 shows one embodiment of our invention,

Fig. 2 is another embodiment of our invention,

and

Fig. 3 is a screen arrangement.

Referring to Fig. 1, there is shown the normal cathode ray tube arrangement comprising an evacuated envelope containing a neck portion.

and a frustro-conical shape section; and I0 is the so-called electron gun or cathode ray generating structure therein; II is an accelerating anode; and I2 are pairs of deflecting plates for controlling the position of the cathode ray on the screen; I3 is a layer of metal deposited on the front wall of the tube and as has been hereinbefore explained, this metal is so chosen that its color when viewed by transmitted light matches or approximately matches the light emitted by the fluorescent material when activated. A lead wire I4 is joined to the metal deposit for supplying a positive potential thereto in order to make the deposit act as an anode. A layer of fluorescent material I5 is then placed on the metallic deposit.

Fig. 2 shows a modification of Fig. 1 in which a wall coating I6 is arranged to overlap the metal deposit l3 and the conductor I4 is brought out through the neck of the tube and joins the wall coating instead of the metal on the front end of the tube on which the fluorescent screen is formed.

Referring to Fig. 3, there is shown a screen arrangement as hereinbefore described. A mica backing I! has deposited thereon the matching metal l3 and deposited on the metal is the fluorescing material 15. For the sake of clarity, the fluorescing material has not been shown as covering the entire metallic deposit although in actual manufacture this probably would be done. There has been illustrated one form of spacing and supporting arm l8 by means of which the mica backing is held in the tube and spaced therefrom. It will be appreciated, of course, that some of the types of mica spacers now commonly in use in thermionic tubes may be used and such has been illustrated here. However, the edge of the mica backing itself might be so formed as to comprise a spacer arm. For the sake of simplicity, only one spacer and supporter has been shown.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we

declare that what we claim is:

1. A cathode ray tube including an envelope, means for generating electrons, means for utilizing said electrons to trace an image representation, a screen member adjustable along the major axis of said tube, said screen member comprising a substantially rectangularly shaped mica support, a layer of a metal having sub.- stantially the same color as the reproduced light, a fluorescing material deposited on said metallic layer, and a plurality of resilient members holding and entirely supporting said mica, said members engaging the envelope wall of the tube and being held in frictional contact thereto, said members engaging the screen only at the diagonally disposed points thereof.

2. Apparatus in accordance with claim 1 wherein the layer of metal having substantially the same color as the reproduced light comprises a thin conductive layer of gold.

GEORGE FAIRBURN BRETT. GEORGE BALDWIN BANKS. 

